Photomask-forming glass substrate and making method

ABSTRACT

A photomask-forming glass substrate having a square major surface is provided wherein two strip regions are defined on the major surface near a pair of opposed sides such that each region spans between 2 mm and 10 mm inward of the side and excludes end portions extending 2 mm inward from the opposed ends of the side, a least squares plane is computed for each of the two strip regions, the angle included between normal lines to the least squares planes of two strip regions is within 10 seconds, and the height difference between two strip regions is up to 0.5 μm.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a Divisional of copending application Ser. No.12/964,762 filed on Dec. 10, 2010, which claims priority to ApplicationNo. 2009-281698 filed in Japan, on Dec. 11, 2009. The entire contents ofall of the above applications is hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to glass substrates for forming photomasks usedin the manufacture of semiconductor-related electronic materials in theadvanced applications, and a method for preparing the same.

BACKGROUND ART

In unison with the continuous advance toward higher integration ofsemiconductor devices, the photolithography process encounters anincreasing demand for further miniaturization. With respect to theflatness of silica glass substrates for forming photomasks, even aphotomask using a substrate having a satisfactory value of flatness hasa likelihood that when mounted in an exposure tool by a vacuum chuck orholding means, the overall surface topography (or shape) of thephotomask may be substantially deformed depending on the surfacetopography of portions of the substrate corresponding to the holdingmeans of the exposure tool.

A photomask is generally prepared by depositing a light-shielding filmon a silica glass substrate and patterning the film. Upon lithographicexposure, the photomask is often held horizontal by chucking thephotomask surface at its outer peripheral portions by a vacuum chuck orholding means. The overall surface topography of the photomask may besubstantially deformed depending on the surface topography of portionsof the substrate corresponding to the holding means of the exposuretool. For this reason, attempts were made to select a substrate havingminimal deformation and to bevel a substrate near chamfered surfaceportions so as to minimize deformation of a pattern-bearing centralregion of the substrate.

For instance, JP-A 2003-050458 discloses that a substrate is judged on apass/fail basis by measuring the flatness of the substrate andsimulating the shape of a photomask substrate after vacuum chucking. Ifmore substrates fail, the manufacture process suffers from substantialwastes. JP-A 2004-029735 refers to the flatness of a substrate, but notto the chucking in an exposure tool, indicating that the photomask showsan insufficient flatness when mounted in the exposure tool.

WO 2004/083961 proposes a reticle substrate configured to minimize thedeformation of the reticle vacuum chucked in an exposure tool whereinthe shape of a substrate surface extending to the outermost peripherynear the boundary of a chamfered surface is defined. The substrate mustbe inspected by a probe type topography measuring apparatus in order toconfirm the shape outside the range that is measurable by a flatnesstester of optical interference type. This means that the substrate afterpolishing is subject to contact inspection, with an increased risk ofdamages being caused by handling. Productivity is undesirably low due toan increased number of inspection steps.

CITATION LIST

Patent Document 1: JP-A 2003-050458

Patent Document 2: JP-A 2004-029735

Patent Document 3: WO 2004/083961

SUMMARY OF INVENTION

An object of the invention is to provide a photomask-forming glasssubstrate including portions of distortion-free shape corresponding tochucking means of an exposure tool, and a method for preparing the sameat a high productivity.

With regard to a photomask comprising a square-shaped glass substratehaving a pair of major surfaces and a patterned light-shielding film onone surface of the substrate which is held at chucking portions in anexposure tool, the inventors have found that the glass substrate becomesan effective photomask-forming glass substrate suited for thehigh-definition, high-accuracy photolithography when the followingconditions are met. The major surface where the chucking portions aredisposed is delineated by four peripheral sides; two strip regions aredefined on the major surface near a pair of opposed sides such that eachregion spans between 2 mm and 10 mm inward of the side and extendsparallel to the side, but excludes end portions extending 2 mm inwardfrom the longitudinally opposed ends of the side; and a least squaresplane is computed for each of the two strip regions, based on a distancefrom any common reference plane to a coordinate point within the stripregion. The angle included between normal lines to the least squaresplanes of the two strip regions is less than or equal to 10 seconds. Itis assumed that F1 is the least squares plane of one strip region, F2 isthe least squares plane of the other strip region, F3 is a least squaresplane for a major surface region coextensive with the major surface,excluding peripheral portions extending 2 mm inward from the four sides,but including the two strip regions, a plane F3′ parallel to F3 isdisposed such that the center of a strip region-equivalent region of F1and the center of a strip region-equivalent region of F2 may fall on thesame side relative to F3′, and the strip regions have a heightdifference represented by the absolute value |D1−D2| of the differencebetween a distance D1 of a normal line drawn from the center of a stripregion-equivalent region of F1 to plane F3′ and a distance D2 of anormal line drawn from the center of a strip region-equivalent region ofF2 to plane F3′. The height difference between the strip regions is lessthan or equal to 0.5 μm. The glass substrate meeting the foregoingrequirements ensures that when a photomask prepared therefrom is mountedin an exposure tool by a vacuum chuck or holding means, any surfacetopographic change of the overall photomask by chucking is minimized.

The inventors have also found that the photomask-forming glass substrateis obtainable by computing a least squares plane for each of theregions, and locally removing each region in accordance with adifference between the least squares plane of the region and an actualsurface so that the region may become flat; more specifically, bydefining two strip regions on the major surface near a pair of opposedsides such that each region spans between 2 mm and 10 mm inward of theside and extends parallel to the side, but excludes end portionsextending 2 mm inward from the longitudinally opposed ends of the side,defining a major surface region coextensive with the major surface,excluding peripheral portions extending 2 mm inward from the four sides,but including the two strip regions, computing a least squares plane forthe major surface region, based on a distance from any common referenceplane to a coordinate point within the region, comparing the leastsquares plane of the major surface region with an actual surfacetopography of each of the two strip regions to determine a difference,computing a removal amount at a coordinate point within each stripregion in accordance with the difference, and effecting local polishingor etching to remove a surface portion of the substrate in each stripregion corresponding to the removal amount. As used herein, the term“topography” is sometimes simply referred to as “shape”.

The invention provides a photomask-forming glass substrate and a methodfor preparing the same, which are defined below.

[1] In connection with a photomask comprising a square-shaped glasssubstrate having a pair of major surfaces and a patternedlight-shielding film on one surface of the substrate which is chucked atchucking portions in an exposure tool,

the glass substrate wherein

the major surface where the chucking portions are disposed is delineatedby four peripheral sides,

two strip regions are defined on the major surface near a pair ofopposed sides such that each region spans between 2 mm and 10 mm inwardof the side and extends parallel to the side, but excludes end portionsextending 2 mm inward from the longitudinally opposed ends of the side,

a least squares plane is computed for each of the two strip regions,based on a distance from any common reference plane to a coordinatepoint within the strip region,

the angle included between normal lines to the least squares planes ofthe two strip regions is less than or equal to 10 seconds,

on the assumption that F1 is the least squares plane of one stripregion, F2 is the least squares plane of the other strip region, F3 is aleast squares plane for a major surface region coextensive with themajor surface, excluding peripheral portions extending 2 mm inward fromthe four sides, but including the two strip regions, a plane F3′parallel to F3 is disposed such that the center of a stripregion-equivalent region of F1 and the center of a stripregion-equivalent region of F2 may fall on the same side relative toF3′, and the strip regions have a height difference represented by theabsolute value |D1−D2| of the difference between a distance D1 of anormal line drawn from the center of a strip region-equivalent region ofF1 to plane F3′ and a distance D2 of a normal line drawn from the centerof a strip region-equivalent region of F2 to plane F3′,

the height difference between the strip regions is less than or equal to0.5 μm.

[2] The glass substrate of [1] wherein both the strip regions have aflatness of less than or equal to 1.0 μm.[3] The glass substrate of [1] or [2] wherein a central square regioncoextensive with the major surface and excluding peripheral portionsextending 10 mm inward from the four sides has a flatness of less thanor equal to 0.5 μm.[4] The glass substrate of any one of [1] to [3] which is a silica glasssubstrate having four sides of each 152 mm long and a thickness of 6.35mm.[5] A method for preparing a square-shaped glass substrate which is usedto form a photomask by forming a patterned light-shielding film on onesurface thereof, comprising the steps of:

defining two strip regions on the major surface near a pair of opposedsides such that each region spans between 2 mm and 10 mm inward of theside and extends parallel to the side, but excludes end portionsextending 2 mm inward from the longitudinally opposed ends of the side,

defining a major surface region coextensive with the major surface,excluding peripheral portions extending 2 mm inward from the four sides,but including the two strip regions,

computing a least squares plane for the major surface region, based on adistance from any common reference plane to a coordinate point withinthe region,

comparing the least squares plane of the major surface region with anactual surface topography of each of the two strip regions to determinea difference,

computing a removal amount at a coordinate point within each stripregion in accordance with the difference, and

effecting local polishing or etching to remove a surface portion of thesubstrate in each strip region corresponding to the removal amount.

[6] The method of [5] wherein

the two strip regions are parallel to the least squares plane of themajor surface region,

a least squares plane is computed for each of the two strip regions,based on a distance from the common reference plane to a coordinatepoint within the strip region,

on the assumption that F1 is the least squares plane of one stripregion, F2 is the least squares plane of the other strip region, F3 is aleast squares plane for the major surface region, a plane F3′ parallelto F3 is disposed such that the center of a strip region-equivalentregion of F1 and the center of a strip region-equivalent region of F2may fall on the same side relative to F3′, and the strip regions have aheight difference represented by the absolute value |D1−D2| of thedifference between a distance D1 of a normal line drawn from the centerof a strip region-equivalent region of F1 to plane F3′ and a distance D2of a normal line drawn from the center of a strip region-equivalentregion of F2 to plane F3′,

said method further comprising the steps of:

computing a desired topography of each strip region which ensures thatthe height difference between the strip regions is equal to zero (0),and

determining the difference between the least squares plane of the majorsurface region and an actual surface topography of each strip regionfrom the difference between the desired topography of each strip regionand the actual surface topography of each strip region.

[7] The method of [6] wherein the desired topography of each stripregion is the least squares plane of the major surface region.[8] The method of any one of [5] to [7], further comprising the stepsof:

defining a central square region coextensive with the major surface andexcluding peripheral portions extending 10 mm inward from the foursides,

computing a least squares plane for the central square region, based ona distance from the common reference plane to a coordinate point withinthe region,

comparing the least squares plane of the central square region with anactual surface topography of the central square region to determine adifference,

computing a removal amount at a coordinate point within the centralsquare region in accordance with the difference, and

effecting local polishing or etching to remove a surface portion of thesubstrate in the central square region corresponding to the removalamount.

[9] The method of any one of [5] to [8], further comprising the step ofdouble-side polishing after the local polishing or etching step suchthat both the strip regions have a flatness of less than or equal to 1.0μm.[10] he method of any one of [5] to [8] , further comprising the step ofdouble-side polishing after the local polishing or local etching suchthat the central square region has a flatness of less than or equal to0.5 μm.[11] The method of any one of [5] to [8], wherein after the localpolishing or etching step, a polishing step is performed for improvingthe surface quality and defective quality of the substrate surface beingprocessed,

the polishing step being carried out after previously evaluating atopographic change of the strip regions before and after the polishingstep, computing an adjustment removal amount by subtracting an amountcorresponding to the topographic change, and effecting local polishingor etching of each strip region, using the adjustment removal amount asthe amount of a substrate surface portion to be removed during the localpolishing or etching step, thereby removing the substrate surfaceportion.

[12] The method of any one of [5] to [11], wherein the glass substrateis a silica glass substrate having four sides of each 152 mm long and athickness of 6.35 mm.

ADVANTAGEOUS EFFECTS OF INVENTION

The invention provides a glass substrate, typically silica glasssubstrate for forming a photomask used in the photolithography processfor the fabrication of ICs and microelectronic devices. When a photomaskis mounted in a mask stage of an exposure tool by a vacuum chuck orholding means for exposure purpose, any surface topographic change ofthe overall photomask by chucking is minimized. The glass substrate canbe prepared in a productive manner through steps as employed in theconventional inspection process. Since the flatness of the photomask isnot adversely affected by the mounting of the photomask in the exposuretool, a microelectronic fabrication process enabling furtherminiaturization is expectable.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a plan view of one major surface of a glass substrate in whichrelevant regions are defined, FIG. 1A illustrating a major surfaceregion, and FIG. 1B illustrating strip regions and a central squareregion.

FIG. 2 is an exaggerated schematic view illustrating the definition ofthe height of strip regions on the glass substrate.

FIG. 3 is an exaggerated schematic view of a cross-sectional profile ofan exemplary glass substrate surface before and after processing basedon an appropriate removal amount computed, in processing of a pair ofstrip regions on the glass substrate, FIG. 3A illustrating the profilebefore removal and FIG. 3B illustrating the profile after removal.

FIG. 4 is an exaggerated schematic view of a cross-sectional profile ofan exemplary glass substrate surface before and after processing basedon an inappropriate removal amount computed, in processing of a pair ofstrip regions on the glass substrate, FIG. 4A illustrating the profilebefore removal and FIG. 4B illustrating the profile after removal.

FIG. 5 illustrates one exemplary processing mode using a small-sizerotary machining tool.

FIG. 6 illustrates another exemplary processing mode using a small-sizerotary machining tool.

DESCRIPTION OF EMBODIMENTS

A photomask comprising a square-shaped glass substrate having a pair ofmajor surfaces and a patterned light-shielding film on one surface ofthe substrate is held at its chucking portions in an exposure tool. Theinvention pertains to the major surface of the glass substrate where thechucking portions are disposed. The major surface is delineated by fourperipheral sides. Now predetermined regions are defined on the majorsurface including the chucking portions where the photomask is held inthe exposure tool. Specifically, as shown in FIG. 1, two strip regions21 are defined on the major surface 1 near a pair of opposed sides suchthat each region spans between 2 mm and 10 mm inward of the side andextends parallel to the side, but excludes end portions extending 2 mminward from the longitudinally opposed ends of the side. An averageplane, i.e., least squares plane is computed for each of the two stripregions 21, based on a distance from any common reference plane to acoordinate point within the strip region. The least squares planes oftwo strip regions 21 are generally parallel and staggered by a step (orheight difference) of less than or equal to 0.5 μm.

More specifically, least squares planes are defined for a pair of stripregions 21, respectively. The angle included between normal lines to theleast square planes is less than or equal to 10 seconds, so that theexposure surface may be more flat when the photomask is held in theexposure tool. For a pair of strip regions meeting this requirement, itis assumed that F1 is the least squares plane of one strip region, F2 isthe least squares plane of the other strip region, F3 is a least squaresplane for a major surface region coextensive with the major surface,excluding peripheral portions extending 2 mm inward from the four sides,but including the two strip regions, and a plane F3′ parallel to F3 isdisposed such that the center of a strip region-equivalent region of F1and the center of a strip region-equivalent region of F2 may fall on thesame side relative to F3′. Then the strip regions have a heightdifference represented by the absolute value |D1−D2| of the differencebetween a distance D1 of a normal line drawn from the center of a stripregion-equivalent region of F1 to plane F3′ and a distance D2 of anormal line drawn from the center of a strip region-equivalent region ofF2 to plane F3′. The height difference between the strip regions is lessthan or equal to 0.5 μm.

The invention is advantageously applicable to the so-called 6-inchsubstrates dimensioned 152±0.2 mm by 152±0.2 mm by 6.35±0.1 mm. Theglass substrate is preferably a silica glass substrate, i.e., quartzglass substrate.

A starting glass substrate, e.g., a glass substrate subject to localpolishing or etching is measured for surface topography. Althoughmeasurement of a surface topography may be done by any desiredtechniques, the optical interference type measurement is a preferredexample because a high accuracy is desired. In a flatness tester ofoptical interference type wherein coherent (in phase) light, typicallylaser light is directed to and reflected by the surface of a substratewhich is held vertically upright, a difference in height betweensubstrate surface areas is observable as a phase shift of the reflectedlight. From this result, the topography of the substrate surface (or ofeach region) is measurable as a height (Z coordinate) relative to the XYcoordinate system of a reference plane having a flatness of 0 μm. Fromthis information, the least squares plane for each region on thesubstrate surface, the angle and height of a normal line thereto, andthe flatness (TIR: total indicator reading) of each region may becomputed.

It is noted that a flatness may be measured by an optical interferencetype flatness tester Tropel UltraFlat® M200 (Corning Tropel Corp.). Inparticular, Tropel UltraFlat® M200 is characterized by a shallow angleof incident light and a higher sensitivity of interference fringes thana flatness tester FM200 of the earlier generation and ensures that themeasurement region is not affected by reflection from the end face evennear the end of the surface, that is, a surface topography in a rangewithin 2 mm from the end face is measurable at a high accuracy.

The least squares plane and the flatness (TIR) have been established asthe means for evaluating photomask-forming glass substrates and may becomputed from coordinate data measured by a flatness tester. Based onthe least squares planes of two strip regions measured by a flatnesstester, the angle included between normal lines to the two least squaresplanes may be determined. It is assumed for a pair of strip regions thatF1 is the least squares plane of one strip region, F2 is the leastsquares plane of the other strip region, F3 is a least squares plane fora major surface region coextensive with the major surface, excludingperipheral portions extending 2 mm inward from the four sides, butincluding the two strip regions, and a plane F3′ parallel to F3 isdisposed such that the center of a strip region-equivalent region of F1and the center of a strip region-equivalent region of F2 may fall on thesame side relative to F3′. As long as the angle included between normallines to the two least squares planes is generally within 3600 seconds,the absolute value |D1−D2| of the difference between a distance D1 of anormal line drawn from the center of a strip region-equivalent region ofF1 to plane F3′ and a distance D2 of a normal line drawn from the centerof a strip region-equivalent region of F2 to plane F3′ may be defined asthe height difference between the strip regions.

The difference in height between a pair of strip regions might bedefined by drawing a normal line from the center of a stripregion-equivalent region of one least squares plane to the other leastsquares plane, or drawing a normal line from the center of a stripregion-equivalent region of the other least squares plane to one leastsquares plane, and determining the absolute value of the differencetherebetween. In this case, however, even when the angle includedbetween normal lines of the strip regions is 10 seconds, only adifference in height resulting from mutual inclination of the stripregions is 0.67 μm at maximum for a 6-inch square substrate, forexample. Thus this definition is undesired.

FIG. 2 is a schematic view in cross section of a substrate forillustrating a difference in height between strip regions as definedabove. Illustrated in FIG. 2 are a position 3 disposed 2 mm inward fromthe substrate side (end face), a position 4 disposed 10 mm inward fromthe substrate side (end face), a substrate surface 5 prior to localpolishing or etching, a least squares plane 6 for a major surfaceregion, a plane 61 parallel to the least squares plane 6 for a majorsurface region, a least squares plane 7 for one strip region, a leastsquares plane 71 for the other strip region, a normal line 8 drawn fromthe center of a strip region-equivalent region of the least squaresplane of one strip region to a plane parallel to the least squares planeof the major surface region, and a normal line 81 drawn from the centerof a strip region-equivalent region of the least squares plane of theother strip region to a plane parallel to the least squares plane of themajor surface region.

In a preferred embodiment, the angle included between normal lines tothe respective least squares planes of two (paired) strip regions iswithin 5 seconds. In another preferred embodiment, the step or heightdifference between two strip regions is less than or equal to 0.25 μm.

In a preferred embodiment, two (paired) strip regions both have aflatness of less than or equal to 1.0 μm, specifically less than orequal to 0.3 μm, and more specifically less than or equal to 0.1 μm. Alarger flatness value may cause the substrate to be inclined or deformedin surface topography upon mounting of the photomask in the exposuretool, even if the angle included between normal lines to the leastsquares planes is within the range.

In a further preferred embodiment, a central square region coextensivewith the major surface and excluding peripheral portions extending 10 mminward from the four sides has a flatness of less than or equal to 0.5μm, more preferably less than or equal to 0.25 μm.

In the practice of the invention, a photomask-forming glass substratehaving the surface topography defined above may be obtained using localetching by a plasma etching technique or local polishing by a small-sizerotary machining tool.

The method for processing the substrate surface into the surfacetopography defined above by local etching or polishing may comprise thesteps of:

(1) defining two strip regions on the major surface near a pair ofopposed sides such that each region spans between 2 mm and 10 mm inwardof the side and extends parallel to the side, but excludes end portionsextending 2 mm inward from the longitudinally opposed ends of the side,defining a major surface region coextensive with the major surface,excluding peripheral portions extending 2 mm inward from the four sides,but including the two strip regions, computing a least squares plane forthe major surface region, based on a distance from any common referenceplane to a coordinate point within the region, comparing the leastsquares plane of the major surface region with an actual surfacetopography of each of the two strip regions to determine a difference,computing a removal amount at a coordinate point within each stripregion in accordance with the difference, and effecting local polishingor etching to remove a surface portion of the substrate in each stripregion corresponding to the removal amount; and optionally, further(2) defining a central square region coextensive with the major surfaceand excluding peripheral portions extending 10 mm inward from the foursides, computing a least squares plane for the central square region,based on a distance from the common reference plane to a coordinatepoint within the region, comparing the least squares plane of thecentral square region with an actual surface topography of the centralsquare region to determine a difference, computing a removal amount at acoordinate point within the central square region in accordance with thedifference, and effecting local polishing or etching to remove a surfaceportion of the substrate in the central square region corresponding tothe removal amount. Local etching by a plasma etching technique or localpolishing by a small-size rotary machining tool may be performed withmethod (1) alone or methods (1) and (2) combined. Each of methods (1)and (2) may be performed only one time or two or more times, whilemethods (1) and (2) may be alternately performed.

The synthetic quartz glass substrate suited as the starting glasssubstrate in the invention is one which has been prepared from asynthetic quartz glass ingot by shaping, annealing, slicing, lapping,and rough polishing. The synthetic quartz glass substrate which has beenflattened to some extent by any conventional well-known polishingtechnique is preferred.

In order to provide the glass substrate with strip regions of thepredetermined or desired topography, preferably with strip regions and acentral square region of the predetermined or desired topography, thesubstrate surface is subjected to local etching by a plasma etchingtechnique or local polishing by a small-size rotary machining tool.

In the embodiment wherein a pair of strip regions are subject to localetching or polishing, the local etching or polishing may be performed bycomputing a least squares plane for the major surface region based on adistance from any common reference plane to a coordinate point withinthe region, comparing a plane which is parallel to the least squaresplane of the major surface region and below the lowest point within anactual surface topography of each strip region (i.e., stripregion-equivalent region within a plane parallel to the least squaresplane of the major surface region) with the actual surface topography ofeach strip region to determine a difference therebetween, computing aremoval amount at a coordinate point within the strip region inaccordance with the difference, and using the removal amount as theremoval amount during the local etching or polishing. In this event, itis preferred that both the two strip regions are parallel to the leastsquares plane of the major surface region; and for the least squaresplanes which are computed for the two strip regions, based on a distancefrom the common reference plane to a coordinate point within the region,the desired topographies of the two strip regions are computed such thatthe height of the two least square planes (which corresponds to thedistance of a normal line drawn from the center of the stripregion-equivalent region of the least square plane of one strip regionto the least square plane of the other strip region) is 0, and adifference between the desired topography of the strip region and theactual surface topography is determined. In a simplified version, theleast squares plane of the major surface region may be applied as thedesired topography of the strip region.

FIG. 3 schematically illustrates in cross section a substrate in whichsubstrate surface portions of two strip regions have been removed inthis way. FIG. 3A is the profile prior to removal, and FIG. 3B is theprofile after removal. In FIG. 3A, 62 and 63 designate a stripregion-equivalent region within a plane parallel to the least squaresplane of the major surface region. In FIG. 3B, 9 designates a substratesurface after local polishing or etching. The remaining components aredesignated by like reference characters as in FIG. 2, and theirdescription is omitted herein. As seen in the cross-sectional profileafter removal, the angle included between normal lines to the leastsquares planes of a pair of strip regions and the height differencebetween the strip regions are improved, and even the flatness isimproved.

Notably, in the embodiment wherein a pair of strip regions are subjectto local etching or polishing, it is not recommended that the differencebetween a plane which is parallel to the least squares plane of each ofpaired strip regions and below the lowest point within the paired stripregions and the actual surface topography of each of the strip regionsis used as the amount of material to be removed during the local etchingor polishing. The reason is that if the angle included between normallines to the least square planes of the paired strip regions prior toremoval is significantly different, the angle included between normallines may not be improved to the desired value after removal.

FIG. 4 schematically illustrates in cross section a substrate in whichsubstrate surface portions of two strip regions have been removed inthis way. FIG. 4A is the profile prior to removal, and FIG. 4B is theprofile after removal. In FIG. 4A, 72 designates a plane parallel to theleast squares plane of the strip region. The remaining components aredesignated by like reference characters as in FIGS. 2 and 3, and theirdescription is omitted herein.

Likewise, in the embodiment wherein the central square region is subjectto local etching or polishing, the local etching or polishing may beperformed by computing a least squares plane for the central squareregion, based on a distance from the common reference plane to acoordinate point within the region, comparing the least squares plane ofthe central square region with an actual surface topography of thecentral square region to determine a difference, computing a removalamount at a coordinate point within the central square region inaccordance with the difference, and using the removal amount as theamount of material to be removed during local polishing or etching.

The removal amount in each region may be assigned to a surface portionof the substrate which is spaced apart from the substrate body when cutby the least squares plane of each region (a portion where an actualsurface is convex or protrudent with respect to the least squares planeis relevant). The removal amount may also be assigned to a surfaceportion of the substrate which is spaced apart from the substrate bodywhen cut by the least squares plane which has been translated to thelowest point of the actual surface (the entire substrate surfaceexcluding the lowest point is relevant).

In the invention, local etching treatment or local polishing treatmentis made so that the substrate surface (strip regions and central squareregion) may take the above-described topography, especially so that thestrip regions, preferably the strip regions and central square regionmay become flatter, and so that a local etching amount or a localpolishing amount by a small-size rotary machining tool or the like maybe increased or decreased in accordance with the topography of each ofthe strip regions and central square region, while the etching orpolishing amount is locally varied at individual sites on the substratesurface. In this event, if the predetermined topography is not availablefrom single local etching or polishing, for example, the local etchingor polishing treatment may be performed plural times or both thetreatments be combined. For example, if either one of strip region andcentral square region fails to take the predetermined topography afterthe local etching or polishing treatment, then that one region may besubjected again to local etching or polishing treatment. If both regionsfail to take the predetermined topography, then both the regions may besubjected again to local etching or polishing treatment.

Specifically, if both of strip region and central square region fail totake the predetermined topography, then local etching treatment byplasma etching or local polishing treatment by a small-size rotarymachining tool is preferably carried out by combining methods (1) and(2) whereby substantially the entirety of the substrate surface issubjected to local etching or polishing treatment. Once a surfacetopography is measured, local etching or polishing treatment is carriedout by either one of methods (1) and (2) or a combination thereof. Forexample, if only the strip region fails to take the predeterminedtopography after the local etching or polishing treatment, then localetching or polishing is carried out again by method (1). If only thecentral square region fails to take the predetermined topography afterthe local etching or polishing treatment, then local etching orpolishing is carried out again by method (2). If both fail to take thepredetermined topography, then local etching or polishing is carried outagain by a combination of methods (1) and (2).

On the other hand, if only the strip region of the starting glasssubstrate fails to take the predetermined topography, then only method(1) is preferably applied to the first local etching or polishing. Ifonly the central square region of the starting glass substrate fails totake the predetermined topography, then only method (2) is preferablyapplied to the first local etching or polishing.

In one embodiment of the invention wherein plasma etching is effected inaccordance with a removal amount computed for each region on the basisof coordinate data measured by a flatness tester as mentioned above, aplasma-generating housing is placed above the selected surface site tobe removed, an etching gas is flowed, and then neutral radicals createdwithin the plasma attack the glass substrate surface in isotropy wherebythe attacked area is etched. The portion where the plasma-generatinghousing is not placed and hence, no plasma is generated is not etchedeven if the etching gas impinges the portion. This enables local plasmaetching.

The moving speed of the plasma-generating housing is determined inaccordance with the surface topography of the starting glass substrate.An etching amount may be controlled by controlling the moving speed soas to be slow at the site requiring a large removal amount and to befast at the site requiring a small removal amount. When theplasma-generating housing is moved on the starting glass substrate, themoving speed of the plasma-generating housing is controlled inaccordance with a necessary removal amount on the surface of thestarting glass substrate, whereby the substrate can be processed to thedesired topography.

It is noted that the method of controlling the moving speed of theplasma-generating housing in accordance with a necessary removal amountof a surface portion of the glass substrate is preferably acomputer-aided control method. Since the movement of theplasma-generating housing is relative to the substrate, either theplasma-generating housing or the substrate itself may be moved.

The plasma-generating housing may be based on any desired system, forexample, a system wherein a glass substrate is disposed between a pairof electrodes, high-frequency electricity is applied to generate aplasma between the substrate and the electrode, and etching gas ispassed through the plasma to create radicals, or a system whereinetching gas is passed through a waveguide tube, microwave is oscillatedto generate a plasma, and a stream of radicals thus created is appliedto the substrate surface. Either of these systems may be employed.

The etching gas is selected in accordance with the type of glasssubstrate, and preferably from a halogen compound gas and a gas mixturecomprising halogen compounds in the case of a silica glass substrate.Suitable halogen compounds include methane tetrafluoride, methanetrifluoride, ethane hexafluoride, propane octafluoride, butanedecafluoride, hydrogen fluoride, sulfur hexafluoride, nitrogentrifluoride, carbon tetrachloride, silicon tetrafluoride, methanetrifluoride chloride, and boron trichloride. A gas mixture of two ormore halogen compounds and a gas mixture of a halogen compound and aninert gas such as argon are also useful.

The surface of the glass substrate as plasma etched may be roughened orinclude a work damaged layer depending on the plasma etching conditions.If so, polishing may be performed for an extremely short time having noor little substantial impact on the flatness, after the plasma etching.The polishing may be by any well-known polishing techniques such asbatchwise rotary double-side polishing and single-wafer rotarysingle-side polishing.

In the embodiment of the invention wherein local polishing by asmall-size rotary machining tool is performed in accordance with theremoval amount in each region computed on the basis of coordinate datameasured by the flatness tester, once the working (polishing) portion ofthe small-size rotary machining tool is contacted with the surface ofthe starting glass substrate, the working portion is rotated and scannedacross the surface to perform polishing of the substrate surface. Thesmall-size rotary machining tool may be any desired one as long as itsworking portion is a rotating member having a polishing ability.Exemplary small-size rotary machining tools include a small-sizepolishing plate which is placed immediately above the substrate, forcedperpendicularly against the substrate under pressure, and rotated aboutan axis perpendicular to the substrate surface, and a rotary workingtool mounted on a small-size grinder wherein the tool is obliquelyforced against the substrate under pressure. With respect to thematerial of the machining tool, at least its working portion comprisesgreen silicon carbide (GC) abrasive, white fused alumina (WA) abrasive,diamond abrasive, cerium abrasive, cerium pad, rubber-bonded compact,felt buff, polyurethane or other materials capable of machining offworkpieces. The working portion of the rotary tool may have any desiredshape selected from circular or disk, annular, cylinder, cannonball, andbarrel shapes.

For machining, the area of contact between the machining tool and thesubstrate is important. The contact area is specifically 1 to 500 mm²,preferably 2.5 to 100 mm², and more preferably 5 to 50 mm². In the eventthat a protrudent portion is a fine undulation of spatial wavelength, alarger substrate contact area leads to polishing of even a regionoutside the protrudent portion of interest, failing to eliminate theundulation and causing to disrupt the flatness. In the event that asurface of a substrate near the end is machined, a larger tool isinconvenient in that if the tool is partially moved outside thesubstrate, the remaining contact portion of the substrate may receive anincreased pressure, which makes flat machining difficult. If the contactarea is too small, an excess pressure may be applied to cause damages,and the travel distance on the substrate may be increased so that apartial polishing time may become long.

When polishing is performed by bringing the small-size rotary machiningtool in contact with a surface portion of protrudent site, thismachining is preferably carried out in the co-presence of an abrasiveslurry. When the small-size rotary machining tool is moved over thesubstrate, any one or more of the moving speed, rotational speed andcontact pressure of the machining tool may be controlled in accordancewith the height of protrusion on the surface of the starting glasssubstrate. Then, a glass substrate having a high flatness is obtainable.

Suitable abrasive grains include silica, ceria, alundum, white alundum(WA), alumina/zirconia, zirconia, SiC, diamond, titania, and germania.They preferably have a particle size of 10 nm to 10 μm and are typicallyused as water slurry. The moving speed of the machining tool may varyover a wide range while it is generally selected in a range of 1 to 100mm/s, but not limited thereto. The working portion of the machining toolis preferably operated at a rotational speed of 100 to 10,000 rpm, morepreferably 1,000 to 8,000 rpm, even more preferably 2,000 to 7,000 rpm.A lower rotational speed may lead to a slow machining rate, requiring alonger time until the substrate is machined to completion. A higherrotational speed may lead to a fast machining rate or a more tool wear,making flatness control difficult. Preferably the working portion of themachining tool is contacted with the substrate under a pressure of 1 to100 g/mm², more preferably 10 to 100 g/mm². A lower pressure may lead toa slow machining rate, requiring a longer time until the substrate ismachined to completion. A higher pressure may lead to a fast machiningrate to make flatness control difficult, and cause noticeable damageswhen foreign particles are introduced into the tool or slurry.

The control of the moving speed of the partial machining (polishing)tool in accordance with the height of protrusion on the surface of thestarting glass substrate may be achieved using a computer. Since themovement of the machining tool is relative to the substrate, thesubstrate itself may be moved instead. With respect to the movingdirection of the machining tool, the structure may be configured to movethe tool in any of X and Y directions based on an imaginary XY plane onthe substrate surface. This structure will be described with referenceto FIGS. 5 and 6. A rotary machining tool 42 is obliquely contacted witha substrate 41. It is assumed that the tool has an axis of rotation 43,and a line or direction 44 of the axis 43 projected on the substratesurface is X axis on the substrate surface. As shown in FIG. 6, therotary machining tool is scanned forward in X axis direction while itsmovement in Y axis direction is fixed. At the time when the tool reachesthe side end of the substrate, the tool is shifted at a fine pitch 45 inY axis direction. Again the tool is scanned backward in X axis directionwhile its movement in Y axis direction is fixed. This scanning operationis repeated until the overall substrate surface is polished. It ispreferred that polishing be done while the rotation axis 43 of therotary machining tool 42 is oblique to a normal to the substrate 41.Specifically the angle of the rotation axis 43 of the tool 42 relativeto a normal to the substrate 41 is in a range of 5° to 85°, preferably10° to 80°, and more preferably 15° to 60°. If this angle is more than85°, then the contact area becomes larger, and it becomes difficult dueto the structure to apply uniform pressure to the entire contact surfaceand hence to control the flatness. If the angle is less than 5°, thenthe tool is approximately perpendicular to the substrate, with alikelihood that the profile as machined becomes worsened and overlappingof such profiles at a constant pitch may not result in a flat surface.

The method of contacting the small-size machining tool with thesubstrate may be a method of adjusting the tool at a height to contactthe substrate and operating the tool at the fixed height, or a method ofcontrolling pressure by pneumatic control so as to contact the tool withthe substrate. The latter method of contacting the tool with thesubstrate by maintaining a constant pressure is preferred because thepolishing speed becomes constant. The former method of contacting thetool with the substrate by maintaining a constant height sometimes failsin flattening because the working portion of the tool gradually variesin size during machining operation due to wear, and consequently, thecontact area or pressure varies to alter the processing rate duringmachining operation.

With respect to the mechanism of flattening a convex or protrudentportion of the substrate surface in accordance with its height, theabove illustrated embodiment is a method of achieving flattening bymaintaining constant the rotational speed of the machining tool and thecontact pressure of the tool with the substrate surface and changing themoving speed of the tool in a controlled manner. In another embodiment,a method of achieving flattening by changing the rotational speed of themachining tool and the contact pressure of the tool with the substratesurface in a controlled manner.

The substrate as polished above may have a flatness F₂ of 0.01 to 0.5μm, preferably 0.01 to 0.3 μm (F₁>F₂).

After the substrate surface is processed by the machining tool,single-substrate polishing or double-side polishing may be performed toimprove the surface quality and defective quality of the final finishedsurface. In the embodiment wherein the finish polishing step isperformed, after the processing of the substrate surface by themachining tool, for the purpose of improving the surface quality anddefective quality of the machined surface, the preferred procedure is asfollows. A removal amount to be polished by a small-size rotarymachining tool is previously determined by taking into account a shapechange resulting from the finish polishing step, polishing by themachining tool is done in the predetermined removal amount, and thefinish polishing step is then performed. Then the finally finishedsurface may be provided with the desired topography and high surfacecompleteness at the same time.

More particularly, the surface of the glass substrate as processed abovemay sometimes be roughened or include a work affected layer, dependingon partial polishing conditions even when a soft machining tool is used.In such a case, very brief polishing may be effected to such an extentas to have no substantial impact on the flatness after partialpolishing.

Use of a hard machining tool, on the other hand, may sometimes lead to arelatively high degree of surface roughening or a work affected layer ofsubstantial depth. In such a case, it may be effective to predict howthe surface shape changes in accordance with polishing parameters of thesubsequent finish polishing and to control the shape of partialpolishing so as to offset the change. For example, if it is predictedthat the overall substrate becomes convex or protrudent by thesubsequent finish polishing step, the partial polishing step iscontrolled so as to previously finish to a concave shape. Then thesubstrate surface may be finished to the desired shape by the subsequentfinish polishing step.

Changes of surface shape which would be induced by the subsequent finishpolishing step, for example, the polishing step for improving thesurface quality and defective quality of the machined surface, arepreviously evaluated by providing a preliminary substrate, and measuringthe surface shape of the preliminary substrate before and after finishpolishing step, using a surface topography measuring apparatus. Based onthe data, it is analyzed by a computer how the shape changes. Then thetarget shape is a shape having added a shape change inverse to the shapechange of the finish polishing step. Removal of a surface portion of thesubstrate by local polishing or etching is carried out in a controlledmanner so that the glass substrate may have the target shape. Then thefinal finished surface becomes closer to the desired shape.

For example, as long as the strip region is concerned, the method mayinvolve computing an adjustment removal quantity from which an amountcorresponding to a topographic (shape) change of the finish polishingstep has been subtracted, removing a substrate surface portion usingthis adjustment removal quantity as the removal amount of a substratesurface portion during local polishing or etching of the strip region,and thereafter, performing the finish polishing step.

The finish polishing step following the local polishing or etching stepmay preferably be double-side polishing. At the end of this polishingstep, both the two strip regions preferably have a flatness of less thanor equal to 1.0 μm, and the central square region preferably has aflatness of less than or equal to 0.5 μm.

The glass substrate thus obtained is used, for example, in themanufacture of a photomask blank by depositing a light-shielding film ofchromium or the like on the glass substrate. The photomask blank is thenprocessed by coating a resist on the light-shielding film, printing adesired pattern in the resist using electron beam or the like,developing the resist film, and etching the light-shielding film throughthe resist pattern. This results in a photomask havinglight-transmitting areas and light-shielding areas defined therein. Thephotomask is mounted in an exposure tool or stepper where a resist filmcoated on a silicon wafer is exposed imagewise. Subsequent processing ina standard way completes fabrication of a semiconductor device. Thephotomask manufactured using the glass substrate of the invention hasthe advantage that when held in the exposure tool by a vacuum chuck orholding means, the photomask undergoes a minimal change of its overallsurface topography and maintains a high flatness.

As described above, after the glass substrate is subjected to localetching or local polishing by a small-size machining tool, a photomaskblank is manufactured by depositing a light-shielding film of chromiumor the like thereon. At this point, the surface shape of the glasssubstrate may be altered by the stress in the light-shielding film ofchromium or the like. In such a case, by predicting how the surfaceshape changes by the film stress, and controlling the shape after localetching or local polishing by a small-size machining tool to a shapeoffsetting the change, a photomask blank whose surface shape is thedesired shape may be manufactured.

EXAMPLE

Examples of the invention are given below by way of illustration and notby way of limitation.

Example 1

A quartz substrate having a square major surface of 152 mm by 152 mm anda thickness of 6.4 mm was prepared. The surface of this quartz substratewas first measured by a flatness tester of optical interference type,finding that the angle included between normal lines to the leastsquares planes of a pair of strip regions was 19.62 seconds and theheight difference between the two strip regions was 0.87 μm. It is nowassumed for a pair of strip regions that F1 is the least squares planeof one strip region, F2 is the least squares plane of the other stripregion, F3 is a least squares plane for a major surface regioncoextensive with the major surface, excluding peripheral portionsextending 2 mm inward from the four sides, but including the two stripregions, and a plane F3′ parallel to F3 is disposed such that the centerof a strip region-equivalent region of F1 and the center of a stripregion-equivalent region of F2 may fall on the same side relative toF3′. Then the height difference between the two strip regions isrepresented by the absolute value |D1−D2| of the difference between adistance D1 of a normal line drawn from the center of a stripregion-equivalent region of F1 to plane F3′ and a distance D2 of anormal line drawn from the center of a strip region-equivalent region ofF2 to plane F3′. A center square region had a flatness of 0.338 μm.

Plasma etching was performed on the strip regions after computing adifference between the least squares plane of the major surface regioninclusive of strip regions and an actual surface of each strip regionand determining a necessary removal amount from the difference.

A plasma-generating housing of high frequency operation (150 W) having acylindrical electrode of 75 mm diameter was used. Sulfur hexafluoridewas used as the etching gas and fed at a flow rate of 100 sccm. Theplasma-generating nozzle was spaced a distance of 2.5 cm from the glasssubstrate. The processing speed under these conditions was previouslymeasured to be 3.2 mm/min. The moving speed of the nozzle was 20 mm/secat a substrate portion corresponding to the lowest profile of thesubstrate. The moving speed of the nozzle across a certain substrateportion was obtained by determining a necessary residence time of thenozzle in that substrate portion and computing a speed therefrom.Treatment was done by moving the nozzle at the computed speed.

After the plasma etching, the substrate surface was measured again bythe flatness tester of optical interference type. The angle includedbetween normal lines to the least squares planes of two strip regionswas 4.23 seconds, the height difference between the two strip regionswas 0.13 μm, and the center square region had a flatness of 0.312 μm.

Example 2

A quartz substrate having a square major surface of 152 mm by 152 mm anda thickness of 6.4 mm was prepared. The surface of this quartz substratewas first measured by a flatness tester of optical interference type,finding that the angle included between normal lines to the leastsquares planes of a pair of strip regions was 38.44 seconds and theheight difference between the two strip regions was 0.74 μm. The heightdifference between the two strip regions is represented by the absolutevalue |D1−D2| as in Example 1. A center square region had a flatness of0.542 μm.

Plasma etching was performed on the strip regions after computing adifference between the least squares plane of the major surface regioninclusive of strip regions and an actual surface of each strip regionand determining a necessary removal amount from the difference. Plasmaetching was also performed on the central square region after computinga difference between the least squares plane of the central squareregion and an actual surface of the central square region anddetermining a necessary removal amount from the difference.

A plasma-generating housing of high frequency operation (150 W) having acylindrical electrode of 75 mm diameter was used. Sulfur hexafluoridewas used as the etching gas and fed at a flow rate of 100 sccm. Theplasma-generating nozzle was spaced a distance of 2.5 cm from the glasssubstrate. The processing speed under these conditions was previouslymeasured to be 3.2 mm/min. The moving speed of the nozzle was 20 mm/secat a substrate portion corresponding to the lowest profile of thesubstrate. The moving speed of the nozzle across a certain substrateportion was obtained by determining a necessary residence time of thenozzle in that substrate portion and computing a speed therefrom.Treatment was done by moving the nozzle at the computed speed.

After the plasma etching, the substrate surface was measured again bythe flatness tester of optical interference type. The angle includedbetween normal lines to the least squares planes of two strip regionswas 13.75 seconds, the height difference between the two strip regionswas 0.17 μm, and the center square region had a flatness of 0.040 μm.

Since the angle included between normal lines to the least squaresplanes of two strip regions did not fall in the desired range, plasmaetching was performed again on the strip regions after computing adifference between the least squares plane of the major surface regioninclusive of plasma etched strip regions and an actual surface of eachstrip region and determining a necessary removal amount from thedifference.

Thereafter, the substrate surface was measured again by the flatnesstester of optical interference type. The angle included between normallines to the least squares planes of two strip regions was 2.11 seconds,the height difference between the two strip regions was 0.11 μm, and thecenter square region had a flatness of 0.026 μm.

Example 3

A quartz substrate having a square major surface of 152 mm by 152 mm anda thickness of 6.4 mm was prepared. The surface of this quartz substratewas first measured by a flatness tester of optical interference type,finding that the angle included between normal lines to the leastsquares planes of a pair of strip regions was 51.63 seconds and theheight difference between the two strip regions was 1.48 μm. The heightdifference between the two strip regions is represented by the absolutevalue |D1−D2| as in Example 1. A center square region had a flatness of1.467 μm.

Plasma etching was performed on the strip regions after computing adifference between the least squares plane of the major surface regioninclusive of strip regions and an actual surface of each strip regionand determining a necessary removal amount from the difference. Plasmaetching was also performed on the central square region after computinga difference between the least squares plane of the central squareregion and an actual surface of the central square region anddetermining a necessary removal amount from the difference.

A plasma-generating housing of high frequency operation (150 W) having acylindrical electrode of 75 mm diameter was used. Sulfur hexafluoridewas used as the etching gas and fed at a flow rate of 100 sccm. Theplasma-generating nozzle was spaced a distance of 2.5 cm from the glasssubstrate. The processing speed under these conditions was previouslymeasured to be 3.2 mm/min. The moving speed of the nozzle was 20 mm/secat a substrate portion corresponding to the lowest profile of thesubstrate. The moving speed of the nozzle across a certain substrateportion was obtained by determining a necessary residence time of thenozzle in that substrate portion and computing a speed therefrom.Treatment was done by moving the nozzle at the computed speed.

After the plasma etching, the substrate surface was measured again bythe flatness tester of optical interference type. The angle includedbetween normal lines to the least squares planes of two strip regionswas 12.05 seconds, the height difference between the two strip regionswas 0.41 μm, and the center square region had a flatness of 0.561 μm.

Since both the angle included between normal lines to the least squaresplanes of two strip regions and the flatness of the center square regiondid not fall in the desired ranges, plasma etching was performed againon the strip regions after computing a difference between the leastsquares plane of the major surface region inclusive of plasma etchedstrip regions and an actual surface of each strip region and determininga necessary removal amount from the difference. Plasma etching was alsoperformed on the central square region after computing a differencebetween the least squares plane of the central square region and anactual surface of the central square region and determining a necessaryremoval amount from the difference.

Thereafter, the substrate surface was measured again by the flatnesstester of optical interference type. The angle included between normallines to the least squares planes of two strip regions was 1.94 seconds,the height difference between the two strip regions was 0.11 μm, and thecenter square region had a flatness of 0.031 μm.

Example 4

A quartz substrate having a square major surface of 152 mm by 152 mm anda thickness of 6.4 mm was prepared. The surface of this quartz substratewas first measured by a flatness tester of optical interference type,finding that the angle included between normal lines to the leastsquares planes of a pair of strip regions was 25.45 seconds and theheight difference between the two strip regions was 0.87 μm. The heightdifference between the two strip regions is represented by the absolutevalue |D1−D2| as in Example 1. A center square region had a flatness of0.338 μm.

Polishing was performed on the strip regions after computing adifference between the least squares plane of the major surface regioninclusive of strip regions and an actual surface of each strip regionand determining a necessary removal amount from the difference.

A local polishing apparatus was used comprising a motor, a small-sizerotary machining tool mounted on the motor for rotation, and a pneumaticmeans for forcing the machining tool. The motor was commerciallyavailable as a micro-motor grinder consisting of a motor unit EPM-120and a power unit LPC-120 from Nihon Seimitu Kikai Kosaku Co., Ltd. Themachining tool was commercially available as a cannonball-shaped feltbuff F3620 of 20 mm diameter by 25 mm from Nihon Seimitu Kikai KosakuCo., Ltd. The setup was such that the machining tool was obliquelycontacted with the substrate surface at an angle of about 50° over acontact area of 5.0 mm².

The substrate was processed over the entire surface by operating themachining tool at a rotational speed of 4,000 rpm and a pressure of 30g/mm², and traversing the tool across the workpiece. As shown by thearrow in FIG. 6, machining was performed by continuously moving themachining tool parallel to X axis, with a shifting pitch of 0.5 mm in Yaxis direction. The processing speed under these conditions waspreviously measured to be 1.1 mm/min. The moving speed of the machiningtool was 50 mm/sec at a substrate portion corresponding to the lowestprofile of the substrate. The moving speed of the tool across a certainsubstrate portion was obtained by determining a necessary residence timeof the tool in that substrate portion and computing a speed therefrom.Treatment was done by moving the tool at the computed speed.

After the partial polishing, the substrate surface was measured again bythe flatness tester of optical interference type. The angle includedbetween normal lines to the least squares planes of two strip regionswas 3.72 seconds, the height difference between the two strip regionswas 0.14 μm, and the center square region had a flatness of 0.072 μm.

Example 5

Ten substrates prepared by the same method as in Example 4 were used aspreliminary substrates. A surface topography of each of 10 substrateswas measured by a flatness tester of optical interference type. Thesubstrates were subjected to a final polishing step (double-sidepolishing) using a soft suede-type polishing pad and colloidal silica.After the final polishing, a surface topography of each substrate wasmeasured. A shape (topographic) change by the final polishing step wasevaluated by subtracting the height data of the surface topography ofeach substrate prior to the final polishing step from the height data ofthe surface topography after the final polishing step to determine adifference, and averaging the difference for 10 substrates. While thefinal polishing step using a soft suede-type polishing pad and colloidalsilica tended to convert the substrate surface shape to a convex one,the shape change was a convex shape of 0.134 μm.

Separately, a quartz substrate having a square major surface of 152 mmby 152 mm and a thickness of 6.4 mm was prepared. The surface of thisquartz substrate was first measured by a flatness tester of opticalinterference type, finding that the angle included between normal linesto the least squares planes of a pair of strip regions was 33.13 secondsand the height difference between the two strip regions was 0.47 μm. Theheight difference between the two strip regions is represented by theabsolute value |D1−D2| as in Example 1. A center square region had aflatness of 0.345 μm.

Next, in order to offset a convex shape of 0.134 μm estimated as theshape change during the final polishing step, a concave shape of 0.134μm which is an inverse of a convex shape of 0.134 μm was added as theestimated shape change during the final polishing step to partialpolishing conditions. Under these conditions, partial polishing by thesmall-size rotary machining tool as in Example 4 was carried out so asto achieve the concave shape. After the partial polishing, the substratesurface was measured again by the flatness tester of opticalinterference type. The angle included between normal lines to the leastsquares planes of two strip regions was 8.15 seconds, the heightdifference between the two strip regions was 0.41 μm, and the centersquare region had a flatness of 0.102 μm.

Next, the final polishing step was carried out under the conditionsemployed for the preliminary substrates. After the final polishing andsubsequent cleaning and drying, the substrate surface was measured bythe flatness tester of optical interference type, finding that the angleincluded between normal lines to the least squares planes of two stripregions was 1.26 seconds, the height difference between the two stripregions was 0.14 μm, and the center square region had a flatness of0.057 μm. The number of defects on an order of 50 nm was 22.

Japanese Patent Application No. 2009-281698 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. In connection with a photomask comprising a square-shaped glasssubstrate having a pair of major surfaces and a patternedlight-shielding film on one surface of the substrate which is chucked atchucking portions in an exposure tool, the glass substrate wherein themajor surface where the chucking portions are disposed is delineated byfour peripheral sides, two strip regions are defined on the majorsurface near a pair of opposed sides such that each region spans between2 mm and 10 mm inward of the side and extends parallel to the side, butexcludes end portions extending 2 mm inward from the longitudinallyopposed ends of the side, a least squares plane is computed for each ofthe two strip regions, based on a distance from any common referenceplane to a coordinate point within the strip region, the angle includedbetween normal lines to the least squares planes of the two stripregions is less than or equal to 10 seconds, on the assumption that F1is the least squares plane of one strip region, F2 is the least squaresplane of the other strip region, F3 is a least squares plane for a majorsurface region coextensive with the major surface, excluding peripheralportions extending 2 mm inward from the four sides, but including thetwo strip regions, a plane F3′ parallel to F3 is disposed such that thecenter of a strip region-equivalent region of F1 and the center of astrip region-equivalent region of F2 may fall on the same side relativeto F3′, and the strip regions have a height difference represented bythe absolute value |D1−D2| of the difference between a distance D1 of anormal line drawn from the center of a strip region-equivalent region ofF1 to plane F3′ and a distance D2 of a normal line drawn from the centerof a strip region-equivalent region of F2 to plane F3′, the heightdifference between the strip regions is less than or equal to 0.5 μm. 2.The glass substrate of claim 1 wherein both the strip regions have aflatness of less than or equal to 1.0 μm.
 3. The glass substrate ofclaim 1 wherein a central square region coextensive with the majorsurface and excluding peripheral portions extending 10 mm inward fromthe four sides has a flatness of less than or equal to 0.5 μm.
 4. Theglass substrate of claim 1 which is a silica glass substrate having foursides of each 152 mm long and a thickness of 6.35 mm.